1. Field of the Invention
The invention relates to a controller for an internal combustion engine.
2. Description of the Related Art
An internal combustion engine mounted on a vehicle, such as an automobile, has been available that performs cylinder deactivation control in which switching between all cylinder operation and cylinder deactivation operation is performed according to operating conditions of the engine. In the all cylinder operation, all the cylinders are operated, and in the cylinder deactivation operation, operation of part of the cylinders is stopped.
In Japanese Patent Application Publication No. 5-163971 (JP-A-5-163971) (in particular, paragraphs [0035] and [0036], FIG. 3), in such cylinder deactivation control, the all cylinder operation is performed when the engine is in high load operation, and the cylinder deactivation operation is performed when the engine is in low load operation. This is because in an internal combustion engine, there is a tendency that the fuel economy is good when the engine is in high load operation, that is, when an operation is performed in which the amount of aft (mixture) taken into the operating cylinder per cycle is large, and it is intended to improve the fuel economy of the engine in consideration of such fuel economy characteristics of the internal combustion engine.
When the engine is in low load operation in which the fuel economy of the internal combustion engine is not good, that is, there is a fear that the amount of air (mixture) taken into the operating cylinder per cycle falls short, the operation of part of the cylinders is stopped by the cylinder deactivation operation, so that the amount of air (mixture) taken into the remaining operating cylinders per cycle is increased. As a result, the amount of air (the amount of mixture) taken into the operating cylinder per cycle in the cylinder deactivation operation becomes closer to the amount of air (the amount of mixture) taken into the operating cylinder per cycle when the engine is in high load operation in the all cylinder operation. Thus, the fuel economy of the internal combustion engine when the engine is in low load operation is improved.
In order to improve the fuel economy of the internal combustion engine, it is preferable that the engine operational region in which the cylinder deactivation operation is performed in the cylinder deactivation control be maximally broadened. However, if the engine operational region in which the cylinder deactivation operation is performed is excessively broadened, depending on the engine operating conditions, combustion conditions can be deteriorated in the operating cylinder that is operating in the cylinder deactivation operation. If the engine operational region in which the cylinder deactivation operation is performed is excessively broadened to the high load side, for example, combustion conditions in the operating cylinder that is operating in the cylinder deactivation operation are deteriorated when the engine operating conditions are in a high load side of such an engine operational region. It is presumed that such deterioration in the combustion conditions in the operating cylinder occurs because the amount of air taken into the operating cylinder per cycle exceeds the amount of air that can be actually taken in when the cylinder deactivation operation is performed under above-described conditions, which influences the combustion of the mixture.
Thus, in the cylinder deactivation control, the engine operational region in which the cylinder deactivation operation is performed is determined in consideration of the above-described circumstances, the cylinder deactivation operation is performed only in the thus determined region, and the all cylinder operation is performed in other region. Thus, the fuel economy is maximally improved by the cylinder deactivation operation, and the deterioration of the combustion conditions caused by performing the cylinder deactivation operation in an inappropriate engine operational region is suppressed.
Meanwhile, some internal combustion engines mounted on vehicles are provided with a variable valve timing mechanism that continuously varies the open/close characteristics of the engine valves, such as intake valves and exhaust valves. In the internal combustion engine provided with such a variable valve timing mechanism, the variable valve timing mechanism is switched between the operation allowed state and the operation inhibited state according to the engine operating conditions. When the variable valve timing mechanism is in the operation allowed state, an operation command value of the variable valve timing mechanism is calculated based on the engine operating conditions, and by operating the variable valve timing mechanism based on the operation command value, the open/close characteristics of the engine valves are made suitable for the current engine operating conditions. On the other hand, when the variable valve timing mechanism is in the operation inhibited state, the variable valve timing mechanism is controlled so that the open/close characteristics of the engine valves are fixed to be the predetermined characteristics.
When an internal combustion engine in which the cylinder deactivation control is performed is provided with the variable valve timing mechanism, if the engine operational region in which the cylinder deactivation operation is performed in the cylinder deactivation control is determined without taking account of the control of engine-valve open/close characteristics that is performed by the variable valve timing mechanism, there is a fear that the following problem concerning the fuel economy and the combustion conditions of the internal combustion engine occurs.
When the variable valve timing mechanism is in the operation allowed state and the engine-valve open/close characteristics are controlled so as to be suitable for the current engine operating conditions, the combustion conditions are kept in good conditions, and it is therefore possible to perform the cylinder deactivation operation in the cylinder deactivation control in a broad engine operational region. When the engine operational region is determined without taking account of such a thing, however, there is a fear that the determined engine operational region is too narrow. In this case, despite the fact that the engine is in operating conditions in which it is possible to perform the cylinder deactivation operation while keeping the combustion conditions at allowable levels, the cylinder deactivation operation is not performed even when the engine is in such operating conditions, which can accordingly reduce the degree of improvement of the fuel economy of the internal combustion engine.
When the variable valve timing mechanism is in the operation inhibited state and the engine-valve open/close characteristics are fixed to be the predetermined characteristics, the engine-valve open/close characteristics are not always suitable for the engine operating characteristics and it cannot be said that the combustion conditions under such a situation is kept in good conditions. For this reason, it becomes difficult to perform the cylinder deactivation operation in the cylinder deactivation control in a broad engine operational region. When the engine operational region is determined without taking account of such a thing, there is a fear that the determined engine operational region becomes excessively broad. In this case, a situation occurs in which the cylinder deactivation operation is performed despite the fact that the engine operating conditions are such that combustion conditions are deteriorated by performing the cylinder deactivation operation, and combustion conditions are therefore deteriorated.